This invention relates generally to spectroscopy measurement techniques, and, more particularly to a sensitive laser spectroscopy measurement system which is capable of attaining measurement sensitivities of less than 10.sup.-4 at the UV, visible and near IR wavelength.
A measurement problem frequently encountered in the laboratory involves the determination of a small signal change from an illuminated detector resulting from a minute change in absorption, gain or scattering of the light in a measurement region which lies between the light source and the radiation sensitive detector. For example, in the analysis of HF/DF chemical lasers the uncertainties in the F-atom concentration in the cavity flow region have hampered chemical efficiency calculation and plagued analytical attempts to model this type of laser. F-atom uncertainties in the cavity region arise both as a result of uncertainties in the F-atom concentration in the plenum and uncertainties introduced by wall recombination of F-atoms in the expansion nozzle. This situation exists for both arc-heated and combustion-heated chemical lasers.
Heretofore various techniques have been utilized to enhance sensitivities of this type of measurement. One such technique by Suchard is Bergerson and presented in an article entitled "Flourine Pressure Change Monitor For a Reacting System", The Review of Scientific Instruments, Volume 43, No. 11, November 1972, pp 1717. This technique utilizes a dual beam, a mechanical chopper and a single photomultiplier detector with an achieved intensity sensitivity measurement capability of 2.5.times.10.sup.-3. In addition, commercial units called ratio meters that measure the ratio of probe to reference beam intensities have been developed by both the ITHACO (model 3512) and P.A.R. (model 188) which utilize laser beam optical sources, mechanical choppers with different frequencies for the reference and probe beams, two phase sensitive lock-in amplifiers and a photomultiplier detector for signal measurement. The sensitivities of these instruments are of the order of 5.times.10.sup.-3.
Consequently, when a need emerges for the measurement of, for example, low F.sub.2 concentration in HF/DF chemical laser flow which requires a sensitivity in the order of less than 10.sup.-4 the techniques of the type listed above fall well short of the required sensitivities.